Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
From: Si-Wei Liu
Date: Mon Feb 22 2021 - 20:13:54 EST
On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
for legacy") made an exception for legacy guests to reset
features to 0, when config space is accessed before features
are set. We should relieve the verify_min_features() check
and allow features reset to 0 for this case.
It's worth noting that not just legacy guests could access
config space before features are set. For instance, when
feature VIRTIO_NET_F_MTU is advertised some modern driver
will try to access and validate the MTU present in the config
space before virtio features are set.
This looks like a spec violation:
"
The following driver-read-only field, mtu only exists if VIRTIO_NET_F_MTU is
set.
This field specifies the maximum MTU for the driver to use.
"
Do we really want to workaround this?
Thanks
And also:
The driver MUST follow this sequence to initialize a device:
1. Reset the device.
2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
3. Set the DRIVER status bit: the guest OS knows how to drive the device.
4. Read device feature bits, and write the subset of feature bits understood by the OS and driver to the
device. During this step the driver MAY read (but MUST NOT write) the device-specific configuration
fields to check that it can support the device before accepting it.
5. Set the FEATURES_OK status bit. The driver MUST NOT accept new feature bits after this step.
6. Re-read device status to ensure the FEATURES_OK bit is still set: otherwise, the device does not
support our subset of features and the device is unusable.
7. Perform device-specific setup, including discovery of virtqueues for the device, optional per-bus setup,
reading and possibly writing the device’s virtio configuration space, and population of virtqueues.
8. Set the DRIVER_OK status bit. At this point the device is “live”.
so accessing config space before FEATURES_OK is a spec violation, right?
It is, but it's not relevant to what this commit tries to address. I
thought the legacy guest still needs to be supported.
Having said, a separate patch has to be posted to fix the guest driver
issue where this discrepancy is introduced to virtnet_validate() (since
commit fe36cbe067). But it's not technically related to this patch.
-Siwei
Rejecting reset to 0
prematurely causes correct MTU and link status unable to load
for the very first config space access, rendering issues like
guest showing inaccurate MTU value, or failure to reject
out-of-range MTU.
Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
---
drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
1 file changed, 1 insertion(+), 14 deletions(-)
diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index 7c1f789..540dd67 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
return mvdev->mlx_features;
}
-static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
-{
- if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
- return -EOPNOTSUPP;
-
- return 0;
-}
-
static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
{
int err;
@@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
{
struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
- int err;
print_features(mvdev, features, true);
- err = verify_min_features(mvdev, features);
- if (err)
- return err;
-
ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
- return err;
+ return 0;
}
static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)